Ruminant animals, particularly when grazing or forage fed, may require prolonged administration of certain substances, beneficial nutritionally and/or medicinally, such as micronutrients (e.g. copper, selenium) or anthelmintics. Due to the difficulty and inconvenience of feeding substances and administering beneficial substances to grazing or forage fed animals, and to the desirability of individual animals receiving the correct amount of such substances, the practice is commonly employed of giving substances in a physical form, such as ruminal pellets known as boluses, which are retained in the ruminant fore-stomachs for prolonged periods, releasing beneficial substances.
Such substances may be released from the bolus by solution into the partly aqueous contents of the rumen (known as leaching, where the bolus matrix is itself insoluble) by gradual solution of the whole bolus (i.e. the bolus matrix is itself water soluble), or by erosion of the bolus.
It is typical of such boluses that the initial rate of release of the beneficial substance(s) is high, and later rates of release are reduced.
FIG. 1 shows mean plasma inorganic iodine levels in iodine-deficient cattle following administration of one or two wax-coated, leaching convex-surfaced iodine-releasing boluses, compared to untreated control animals. An initial sharp peak in the mean level in treated animals was followed by decline throughout the remaining period of release.
A typical target period of release for such boluses is six months. An ideal pattern of release would be entirely even, supplying a consistent and uniform release of beneficial substances throughout the life of the bolus.
It is known to form a bolus for administration to a ruminant animal using a mixture of three components: one or more beneficial substances to be released from the bolus (such as a mineral supplement or a therapeutic anti-parasitic agent), a ballast material (such as zinc, which increases the density of the bolus and helps to retain the bolus in the rumen), and a binder, such as a resin. It is further known to coat such a bolus with a protective coating of a substance with no, or very low, solubility in water (such as a wax), as described in GB 2,369,298. The protective coating prevents formation during storage of unsightly particles of the oxidised ballast metal, e.g. zinc oxide or iron oxide, and reduces the rate of leaching of water-soluble beneficial substance(s) from the bolus body. The protective coating is gradually abraded or eroded from the surface of the bolus body during the first week in the rumen.
In a variant of the sort of bolus described above, the use of coatings of different thicknesses over different parts of the bolus, and the use of a coating made from different substances at different parts of the bolus, is taught in GB 2,353,707. Both of these procedures increase complexity and cost in manufacturing. The concept of a bolus which may be only partially covered with a protective coating is disclosed in GB 2,333,451. GB 2,376,630 teaches a segmented bolus, which is intended to provide an intermittent, pulsed release of a beneficial substance, rather than a uniform, continuous rate of release.
It is an object of the present invention to provide a bolus which has more consistent and uniform rate of release of a beneficial substance than is obtainable with a conventional bolus.